a) the government should not issue marriage licenses--that is unwarranted meddling in religion.
That is a logical conclusion. It's fair for the state to require registration after the fact, for legal record-keeping, tax purposes, etc., but it's wrong for the government to pretend to be God and grant or withhold permission by "licensing" marriages. No license is required to start a newspaper or print a book; surely marriage is at least as fundamental a human right.
b) any "marriage" performed at the county courthouse without benefit of clergy is null and void.
Only if the religion claims exclusive right to preside over the ceremony. That's certainly not the case of Christianity (the Bible records several marriages performed without clergy: e.g. Issac's marriage to Rebecca (Gen. 24:67), some Christians' mistaken views to the contrary notwithstanding.
c) there should be no such thing as legal marriage between two atheists.
Again, only if the religion claims such; which Christianity does not. The only restrictions of this sort that the Bible places on marriage is that Christians may not marry outside the faith.
d) there should be no such thing as legal marriage, period, since it is a matter of religion rather than secular law.
Again, only if the particular religion says the state has no right to recognize marriage legally, and there's nothing like that in the Bible. The state may, and should, add its recognition of marriage to the biblical one, but it may not legitimately supplant it, e.g., by requiring licenses.